Method and apparatus for improving image clarity and sensitivity in optical coherence tomography using dynamic feedback to control focal properties and coherence gating

ABSTRACT

Methods for optical imaging, particularly with optical coherence tomography, using a low coherence light beam reflected from a sample surface and compared to a reference light beam, wherein real time dynamic optical feedback is used to detect the surface position of a tissue sample with respect to a reference point and the necessary delay scan range. The delay is provided by a tilting/rotating mirror actuated by a voltage adjustable galvanometer. An imaging probe apparatus for implementing the method is provided. The probe initially scans along one line until it finds the tissue surface, identifiable as a sharp transition from no signal to a stronger signal. The next time the probe scans the next line it adjusts the waveform depending on the previous scan. An algorithm is disclosed for determining the optimal scan range.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from provisional application No.60/287,477, filed Apr. 30, 2001, and commonly assigned to the assigneeof the present application, and which is incorporated herein in itsentirety.

FIELD OF THE INVENTION

The present invention relates to methods for optical imaging using a lowcoherence light beam reflected from a sample surface and compared to areference light beam, wherein real time dynamic optical feedback is usedto detect the surface position of a tissue sample with respect to areference point and the necessary delay scan range. The present alsorelates to an imaging probe apparatus for implementing the method.

BACKGROUND

Optical coherence tomography is an imaging technique that measures theinterference between a reference beam of light and a detected beam oflight that has impinged on a target tissue area and been reflected byscatterers within tissue back to a detector. In OCT imaging of bloodvessels an imaging probe is inserted into a blood vessel and a 360degree circular scan is taken of the vessel wall in series of segmentsof a predetermined arc to produce a single cross sectional image. Theprobe tip is rotated axially to create a circular scan of a tissuesection and also longitudinally to scan a blood vessel segment length,thus providing two-dimensional mapped information of tissue structure.The axial position of the probe within the lumen remains constant withrespect to the axial center of the lumen. However, the surface of thewall may vary in topography or geometry, resulting in the variance ofthe distance between the probe tip and the surface. Since conventionalOCT imaging uses a fixed waveform to create the incident light beam in aschematically rectangular “window” of a certain height, the variation insurface height of the wall may result in the failure to gather tissuedata in certain regions of the blood vessel wall. It would desirable tohave a feedback mechanism that would cause the modification of thewaveform to shift the window based on where the probe is and what itsees.

In traditional OCT systems, the length of the scanning line and itsinitial position have always been constant and fixed. One way toovercome this problem is to make the window larger. The problem withthis is that the signal to noise ratio and accompanying sensitivitydecrease because one is collecting information over a larger area in thesame amount of time.

It would be desirable to use the identification of the tissue surface toadjust the starting position of the scan to a different spot. Theidentification of the surface could also be used to adjust the focallocation in the sample arm. It would additionally be desirable if theidentification of the attenuation of light within the tissue were usedto adjust the scan range. The attenuation identification could also beused to determine an optimal depth of focus or confocal parameter.

SUMMARY OF THE INVENTION

The present invention provides methods for optical imaging using a lowcoherence light beam reflected from a sample surface and compared to areference light beam, wherein real time dynamic optical feedback is usedto detect the surface position of a tissue sample with respect to areference point and the necessary delay scan range. The present alsorelates to an imaging probe apparatus for implementing the method. Theprobe initially scans along one line until it finds the tissue surface,identifiable as a sharp transition from no signal to a stronger signal.The next time the probe scans the next line it adjusts the waveformdepending on the previous scan.

The present invention provides a time delay scanning unit as describedherein. The present invention also provides a focus adjusting mechanismfor an optical scanning system. The present invention also provides amethod of time delay scanning to more accurately determine probe totissue surface distance variations due to surface topography and probelength/design.

The present invention provides a rocking mirror, as one of several novelmechanisms, to create the delay line. A rocking mirror can be moved muchfaster and more accurately to retain synchronicity with the computer andthe scanning probe. The present invention provides an algorithm todetermine position to determine the changes to the galvanometric DCoffset angle to conform to tissue distance from the probe tip. Inaddition, the present invention provides dynamic active feedback toalter the galvanometric AC angle to adjust the coherence gate scan depthto contain only useful image information. Finally, the present inventionalso is capable of using dynamic active feedback to adjust the focusingproperties of the catheter (focal length, spot size, and confocalparameter).

These and other objects, features, and advantages of the presentinvention are discussed or apparent in the following detaileddescription of the invention, in conjunction with the accompanyingdrawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the invention will be apparentfrom the attached drawings, in which like reference characters designatethe same or similar parts throughout the figures, and in which:

FIG. 1 is a graph of a seradyne waveform of a conventional DC baselineoffset.

FIG. 2A is a graph of the vessel wall offset contour of one contour scanwaveform.

FIG. 2B is the normal (constant offset) scanning wave of ΔL_(R).

FIG. 2C a graph of the superimposition of the contour ΔL of FIG. 2A ontothe seradyne waveform of FIG. 2B.

FIG. 2D is the compensated reference arm scan over a period of two axialscans e₁ and e₂.

FIG. 3A is a graph of the scan depth control.

FIG. 3B is a cross-sectional representation of the lumen and the scanrange of FIG. 3A.

FIG. 3C is an image of the cross section of an actual scan.

FIG. 4 is a comparison of the traditional OCT image window and a windowusing the present invention.

FIG. 5 is a graph of the initial offset and Δz the useful scan range.

FIG. 6 is a graph of the modified galvanometric waveform mapped toconform the reference arm delay to the tissue surface contour.

FIGS. 7A-C show successive delay scan lines of the reference arm.

FIG. 8A shows the Δx versus ΔL.

FIG. 8B shows time versus L_(R).

FIG. 9 shows a flow diagram of the algorithm according to one embodimentof the present invention.

FIG. 10 shows four possible hits of signal threshold strength andpotential tissue surface boundary.

FIG. 11 shows a scan line.

FIG. 12 shows the array of the output/storage of the galvanometricwaveform to computer memory.

FIG. 13A shows the old and FIG. 13B new window attainable from block 28of FIG. 9.

FIG. 14 shows a flow diagram for an alternative embodiment of thepresent invention providing an autofocus algorithm.

FIG. 15 shows an algorithm for confocal parameter adjustment duringconfocal microscopy analysis.

FIG. 16 shows a schematic of an apparatus according to one embodiment ofthe present invention.

FIG. 17 shows a schematic of the delay line.

FIG. 18 is a schematic diagram of an alternative system in which thedelay line is created by a mirror 84 is reciprocatingly mounted on alinear translator 85.

FIG. 19 is a schematic diagram showing a further alternative system inwhich a drum 65 controlled by a computer 25.

FIG. 20 illustrates an alternative using an acousto-optic modulator.

FIG. 21 shows a catheter according to the present invention.

FIG. 22 shows a detail of a catheter according to one embodiment of thepresent invention.

FIG. 22A shows an inset of FIG. 22 illustrating the movement of the lenswith respect to the fiber tip.

FIG. 23 is a detail of a catheter design incorporating a balloon or anexpansion chamber to control lens-fiber distance offset.

FIG. 24 shows a schematic view of a system for changing focus.

FIG. 25 shows a schematic view of an alternative embodiment system wherethe fiber-lens separation is fixed and the separation between the lensand the reflector/prism is changed.

FIG. 26 shows a schematic view of a system where the gap between thefiber and a compound lens composed of multiple elements.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Offset and Scan Depth Control

FIG. 1 is a graph of a seradyne waveform of a conventional DC baselineoffset, where L_(R) is the reference arm optical delay distance offsetand t is time (e.g., 0-20 kHz). One scan image length is shown as “e₁”and a second is shown as “e₂”. The peak-to-peak amplitude is called theAC component.

FIG. 2A shows a graph of the vessel wall offset contour of one contourscan waveform where the x-axis is time and the y-axis is ΔL. FIG. 2Bshows the normal (constant offset) scanning wave of ΔL_(R) which is aseradyne wave and is shown in where each period is a single scan image(shown as bracketed Axial Scan 1 having a scan image length of e1 andAxial Scan 2 having a scan image length of e2). In a given contour therecan be somewhere in the range of 250-500 seradyne scans. FIG. 2A showsthe offset correction for a period of a single scan. Optical delay (ΔL)is calculated asΔL=ΔL _(S) −ΔL _(R)where ΔL_(S) is the distance of the sample arm to the tissue surface andΔL_(R) is the optical path of the reference arm.

FIG. 2C shows the superimposition of the contour ΔL of FIG. 2A onto theseradyne waveform of FIG. 2B. “A” is the start gate; “B” is the tissueor vessel surface; “C” is the inside tissue; “D” is the end gate; and“e” is the waveform period. FIG. 2D shows the compensated reference armscan over a period of two axial scans e₁ and e₂. A small image window isdesirable to reduce signal to noise level. The scan is started at offset“a” (start gate) which is slightly away from the vessel surface so thatthe vessel surface is at the top of the scan. This is useful inestablishing the initial scan offset (starting measurement) fordetermination of the algorithm (as discussed in detail below). Thedifference between “b” and “a”, expressed as b-a, is the deadspacebetween the outside and the vessel surface. b-c is the area inside thevessel surface. The image window of FIG. 2C can be expressed as d-a.

FIG. 3A is a graph of the scan depth control. FIG. 3B is across-sectional representation of the lumen and the scan range of FIG.3A. The innermost circle is the catheter 1, the next circle outward isthe vessel lumen 2, the next circle outward is the blood vessel wall 3,and the maximum scan range is indicated at 4. The “+” in a circle areais the useful scan range; the − (minus sign) in a circle is beyond theuseful scan range. FIG. 3C is an image of the cross section of an actualscan.

FIG. 4 shows a comparison of the traditional OCT image window, shown asa square labeled 5 (in solid line) and a window obtainable using thealgorithm of the present invention where the image window labeled as 6(in dashed line). The smaller window 6 has much higher signal to noiseratio and therefore provides significantly increased sensitivity,resulting in an improved image quality.

With previous OCT, the scan waveform has a constant AC component and afixed DC, or slowly varying component. With the present invention the ACcomponent of the waveform as well as the DC component vary with thefeedback from the algorithm. See FIG. 5: “D”, the initial offset and Δzthe useful scan range is observed to determine how to modify thewaveform for the next scan. FIG. 6 is a graph of the modifiedgalvanometric waveform mapped to conform the reference arm delay to thetissue surface contour.

FIGS. 7A-C show successive delay scan lines of the reference arm. FIGS.7A1 and 7A2 shows amplitude a₁, and Δz₁. FIGS. 7B1 and 7B2 showamplitude a₂=2×a1 and Δz₂=2×Δz₁. FIGS. 7C1 and 7C2 show amplitudea₃=0.5×a₁ and Δz₃=0.5×Δz₁. The longer the range (Δz), the greater thedelay in the reference arm.

FIG. 8A shows Δx versus ΔL. FIG. 8B shows time versus L_(R). As thedetermined scan range increases, the galvanometric reference arm ACcomponent also increases. The DC offset follows the curve representingthe tissue surface contour, as in FIG. 8B. Note that Scan 1, Scan 2,etc., of FIG. 8A maps onto Scan 1 and Scan 2 of FIG. 8B. Successivescans 3, 4, . . . N are adjusted for tissue surface offset and optimalscan range in a similar manner. Examination of the data in the presentscan line (axial scan) or scan lines determines the offset to the tissuesurface and the optimal coherence gate for the following N scan lines.In this manner, real-time dynamic feedback is provided and enablesimaging of irregular tissue contours with an optimal sensitivity.

Method

FIG. 9 shows a flow diagram of the algorithm according to one embodimentof the present invention. A first scan line is taken at block 10sufficient to find the tissue surface “S” at block 12 at a relativelylarge scan range (block 14) (for example, about 3-10 mm, although otherranges can be used as appropriate). To find the surface one of at leastthree methods can be used. The first method is to use the adaptivethreshold (“T”). The second method uses the first derivativedI(z)/dz=D1. The third method uses the second derivate zero crossing:d²I(z)/dz²=D2.

There are several rules A, B, and C involved. For the first method rule“A” is: if I(z₁)>T, then S=z₁. For the second method, rule “B” is: ifdI(z₂)/dz>T, then=z₂S. For the third method, rule “C” is: ifd²I(z₃)/dz²=0, then=z₃S. Note, I(z) may need to be filtered to removenoise before doing the derivatives and reduce the introduction ofpreprocessing spikes. Such filtration may be achieved using any of anumber of filters known to those skilled in the art, including, but notlimited to, linear blur, Gaussian, windows, low pass filters,convolution, morphology, and the like. If the surface is not found,repeat block 10, but change the range offset based on the results atblock 12. For example, if there is no signal, the offset and range maybe altered in a random manner. If there is a signal but it is weak anddid not exceed an adaptive threshold, the offset is adjusted (i.e., movethe S and gate toward the signal and try again). That offset is madebased on the intensity of reflect light detected by the detector.

There could be a potential problem at block 12 if the sheath plusinternal reflections is catheter based, or signal based, where thehighest signal is inside the tissue. In such a case there may be morethan one location “z” which has the derivatives>T.

In such cases the rules A, B, and C above are parsed to determine whichcorresponds to tissue surfaces. FIG. 10 shows four possible hits. Thereis only one that corresponds to the tissue surface. ε is a smallincrement. Peak “A” shows an isolated hit where there is no appreciablesignal on either side of the peak; therefore, for z_(A)−ε<z_(A)<z_(A)+ε,there is I(z_(A)±ε)<<I(z). Peak “B” shows a peak where there is nosignal before (i.e., to the left) but there is signal after (i.e., tothe right); therefore, for z_(B)−ε<z_(B) there is I(z_(B)−ε)<<I(z_(B))and I(z_(B))≈I(z_(B)+ε). Stated differently, FIG. 10 shows four caseswhere the signal (image data) threshold is exceeded. Peak “A” has nosignal before or after it (i.e., within the next pixel, increment or ε)it (sometimes referred to as above (z₀) or below (z_(max))); therefore,it is discounted. Peak “D” is discounted for the same reason/rule: ithas no signal before or after it. For peak “C” there is signal before itand after it, therefore it cannot be at the surface. For peak “B” thereis signal after it, but not before it. Therefore, peak “B” indicates thestart of the tissue surface boundary.

Referring back to block 14 there is now a fixed range, typically largerthan desired for the first line. FIG. 11 shows a scan line. The optimalscan range R is what is to be determined. First, the curve is smoothed(see methods mentioned above). Then, second, go out to a large z wherethere clearly is no signal; i.e., find where I(z_(max))=noise. This canbe verified by finding where the standard deviation of (I(z±ε)) is low.Third, decrease z (i.e., move z towards S) until I(z) starts to increaseagain; i.e., I(z′)>I(z_(max)) and where R=z′−S.

Another method of achieving a similar result is to first smooth and takethe derivative of the curve and find out where d(I(z′))/dz=0 andtherefore R=z′−S.

Other statistical methods are possible. A basic operating parameter isthat one wants minimal signal outside of and as much signal as possibleinside of the scan range R. This can be achieved by zeroth order, firstderivative, second derivative, probability distribution functionsstatistics (e.g., standard deviation), fitting to exponential and otherstandard data analysis procedures known in the art.

Spikes in noise, but which are artifacts which could be counted in asignal solution can be a potential problem. One can use filters (median,ordered, adaptive, closing, dilitation or other filter known in the art)to eliminate spikes caused by out of range artifacts.

Referring back to FIG. 9, the reference arm delay waveform is modifiedat block 16. There is a known 1:1 relationship between data acquired bythe computer and reference arm position. S and R can be used to modifythe waveform controlling the optical delay line. S and R now need to beinserted into an equation which controls the galvanometric waveform.Thus G(t)=f(S,R,t), where G(t) is the galvanometric waveform and f is afunction. This G(t) is sent digitally or analog to the galvanometricwaveform. FIG. 12 shows the array of the output/storage of thegalvanometric waveform to computer memory block 20 and which goes toremapping at block 28, where “N” is the number of axial scans per image.This S,R array indicates how to remap the data into real space again forblock 28 (of FIG. 9).

FIG. 13A shows the old and FIG. 14B shows the new window attainable fromblock 28 (refer back to FIG. 9 and accompanying description of referenceletters). I(x,z) are inserted into a remapping function with the inputsbeing an array of S, R to create the remapped image of block 28. Forevery line, x, there are different elements, S and R, in the array(i.e., S₀ corresponds to I(x₀,z) and z is continuous. This relates tothe distance between the probe and the chosen range.

Remapping (block 28 of FIG. 9) is preferably done after each scan. Forstorage, the image is remapped after acquisition. For display, remappingis done interactively. Add each S that is known for each of the scanlines (the vertical bars) to the data and the contour is remapped. S isadded to the offset of the image. In other words, shifting the data forany given exposition by S. Each vertical bar gets (axial scan) remapped(shifted) based on their respective S value. For example, is the zvalues in x₁ are offset by S₁.

There are multiple different equations possible for remapping, examplesof which are shown below:I(x _(n) ,z)=I _(acq)(x _(n) ,z−S _(n))  (1)I(x _(n) ,z)=I _(acq)(x _(n) ,z−S _(n−1))  (2)I(x _(n) ,z)=I _(acq)(x _(n) ,z−S _(n+1))  (3)where n identifies a specific axial scan and where n is close to wheremapping is occurring.

One is thus using array R,S to redisplay/remap the image. This is themost efficient way of storing the remapped image. S can bestored+I_(acq)(z) and reconstructed offline. Or, S+I_(acq)(z) can bereconstructed dynamically or interactively.

The output is sent to the reference arm at block 18 and also saved inthe computer at block 20. If the image is not done at block 22, the nextscan line is taken at block 24 by cycling back repeatedly to block 12until the image is acquired. If the image is done, then the image isremapped at block 28 using the surface S information and the modifiedreference arm delay waveform stored and recalled from the computermemory from block 20. The image is then saved or displayed at block 30.If no other image at block 32 is to be taken, the process is done atblock 40.

Optionally, if another image is to be taken at block 32, then thealgorithm queries at block 34 whether a new location is taken. If yes,then at line 36 the first scan line is taken back at block 10. If noimage is scanned at line 38, then the next surface location S is foundat block 12.

Autofocus

In an alternative embodiment the present invention can be used in anautofocus mode. FIG. 14 shows a flow diagram for an autofocus algorithm.

If Sn and Rn are known, then an optimal focal length is also known andthe optimal spot size and confocal parameters can be calculated. If somefunction “g” is applied to the catheter which causes a change in focusby z_(f), and which occurs at pixel “n” where one knows S_(n), then allone needs to know is, if one is at S_(k) then one can calculate how gchanges as (S_(k)-S_(n)). Therefore, for a given n, one knows what onehas to do to the catheter to obtain a focus of z_(f)(n). S_(n) is alsoknown. So, S_(n+1) creates g(n+1) for all n. In other words, S allowsone to adjust the focus so that it is optimally present within or at thesurface of the tissue. R allows one to adjust the confocal parameter sothat the spot size is minimized over the optimal scan range. Thesealterations of the catheter are performed in real-time, using dynamicfeedback obtained from the image. These enhancements enable optimalimaging of the tissue under investigation.

A key feature of the present invention is that one can calculate whereto move the focus if one position is known. One does not have toiteratively modify the focus until it is optimized each time, only once,and, once S is calculated, modify focus thereafter using the previous orpresent S of the scan. The present invention allows imaging of tissuewith an irregular surface and keeping substantially the entire image inview. Moreover, the scan range is decreased so as to only include usefulimage information, therefore decreasing the bandwidth of the signal andincreasing the image sensitivity of even possibly up to some 3-5 times.The sensitivity increase may be implemented by decreasing the bandwidthof the filter used reject noise while performing heterodyne or lock-indetection. This filter bandwidth may be adjusted dynamically by usingdiode switched capacitor arrays. Increasing sensitivity is equivalent toincreasing speed while keeping accuracy. This is important incardiovascular system imaging. Further, increasing speed decreasesmotion artifacts from heartbeat and blood pressure with concomitantlumen expansion and accompanying modulation of the arm-sample distance.Autofocus enables one to place the optimal focus on the tissue for everyscan position in a rapid manner, thus leading to sharper images. Thepresent invention also has the advantage of compensating for probelength variation.

The present invention provides a time delay scanning unit as describedherein. The present invention also provides a focus adjusting mechanismfor an optical scanning system. The present invention also provides amethod of time delay scanning to more accurately determine probe totissue surface distance variations due to surface topography and probelength/design.

Confocal Parameter

FIG. 15 shows an algorithm for confocal parameter adjustment duringconfocal microscopy analysis. The confocal parameter is optimized to R,the optimal scan gate range. After the first scan line is taken at block210, the optimal grating range R (as previously described hereinabove)is determined, block 212. The optimal confocal parameter 2z_(R) iscalculated at block 214. Then the catheter confocal parameter ismodified at block 216 for some 2z_(e)>(R+ε). If the image is not done atblock 218, go to the next scan line 220. If the scan is done, end atblock 222. 2z_(R)=(2πω₀ ²)/λ, where ω₀ is the beam radius; λ iswavelength, and 2z_(R) is the confocal parameter.

Apparatus

FIG. 16 shows a schematic of an apparatus according to one embodiment ofthe present invention. The basic description of this and the subsequentdrawings is found in Ozawa et al., U.S. Pat. No. 6,069,698, which isincorporated herein. The basic description of the relevant parts of FIG.16 corresponds to FIG. 1 of Ozawa et al.

FIG. 17 shows a schematic of the delay line. The galvanometer is a motorthat attaches to the mirror and actuates partial tilt/rotation of themirror. Only one delay is necessary, although more than one delay lineis possible. Alternatively, one can use a diffraction grating having aperiod which changes as a function of time to make the mirror fixed andnot rotating. Simple, blazed, or other grating known to those ofordinary skill in the art, can be used. The grating sends differentwavelengths to a lens and a galvanometric scanning mirror which altersthe optical delay in the reference arm as a function of mirror angle.

FIG. 18 is a schematic diagram of an alternative system in which thedelay line is created by a mirror 84 is reciprocatingly mounted on alinear translator 85 which is controlled by a motor/driving unit 86 and87. A description of basic components FIG. 18 is found in thespecification corresponding to FIG. 11 of Ozawa et al. The mirror 84oscillates at a certain rate. According to the present invention, thealgorithms would have the mirror 84 scan back and forth and graduallyshifts its translation over time to track the surface of the tissue.Each time the mirror 84 scans, it is called one scan or one axis ofprobing.

FIG. 19 (similar to FIG. 6 of Ozawa et al.) is a schematic diagramshowing a further alternative system in which a drum 65 controlled by acomputer 25. Small changes to the diameter of the drum, induced bypiezoelectrics, stretch the thin fibers wound around the drum. Theincreased fiber length contributes a delay line.

FIG. 20 illustrates an alternative using an acousto-optic modulator 153is a computer controlled diffraction grating where the periodicity ofthe grating can be changed based on the frequency to the acousto-opticmodulator.

FIG. 21 shows a catheter according to the present invention, and is amodification of FIG. 4 of Ozawa et al.

FIG. 22 shows a detail of a catheter according to one embodiment of thepresent invention. The design is based on FIG. 4 of Ozawa et al. FIG.22A (a detail of FIG. 21) shows the distal end of the catheter having anoptical fiber fixed into block 49, which fixes the fiber to the spring.Instead of a fixed block 49 the present invention uses a block which canhave its length altered. In one embodiment, the block is a piezoelectrictransducer (“piezo”) 49A connected by a wire 49B. The voltage changesthe length of the piezo 49A and therefore changes the separation (thegap) between the lens 56 and the tip of the optical fiber. Movement ofthe lens with respect to the fiber tip is shown in the inset FIG. 22A.58 is the output beam. 58 a is the output beam at piezo voltage Va and58 b is the output beam at piezo voltage Vb.

There are alternative ways to controllably change the distance betweenthe lens and the fiber tip. One way is by using a balloon or anexpansion chamber instead of the piezo 49. Instead of the wire 49B thereis an air or hydraulic capillary 49C extending in the catheter 8. SeeFIG. 23, where 58 a is the output beam at air or fluid pressure Pa and58 b is the output beam at pressure Pb.

FIGS. 24 and 25 are two general ways to translate a focus. FIG. 24 showsa schematic view of a system which illustrates that as the distancebetween the fiber and the lens changes, the location of the focuschanges. For object distance d₁ the focus is shown as a solid raytracing line. For distance d₂ the focus is shown as the dashed raytracing line. The relationship between distance and focal length is1/d+1/i=1/f, where “i” is the image distance. Magnification M=i/d.

FIG. 25 shows a schematic view of a system where the fiber-lensseparation is fixed and the separation between the lens and thereflector/prism is changed. In this embodiment, the light beam atdistance d1 has a different focal point than the light beam at distanced2. The translation can be achieved by any of the mechanisms describedabove.

FIG. 26 shows a schematic view of a system where the gap between thefiber and a compound lens composed of multiple elements is fixed and,e.g., the gap between the lens and the reflector is fixed, but therelative separation of the gap between individual lens elements changes.An alternative embodiment utilizes a lens having a flexible cover andfilled with an optically transparent fluid (e.g., saline, oil), gas orother substance. As the fluid composition, flexible cover shape or thelike is changed, the focal length also changes.

It will be understood that the terms “a” and “an” as used herein are notintended to mean only “one,” but may also mean a number greater than“one.” While the invention has been described in connection with certainembodiments, it is not intended to limit the scope of the invention tothe particular forms set forth, but, on the contrary, it is intended tocover such alternatives, modifications, and equivalents as may beincluded within the true spirit and scope of the invention as defined bythe appended claims. All patent, applications and publications referredto herein are incorporated by reference in their entirety.

The invention claimed is:
 1. An apparatus for obtaining informationassociated with at least one structure, comprising: at least one opticalcoupler first arrangement configured to receive at least one firstoptical radiation from the at least one structure and at least onesecond optical radiation from a reference; and at least one computersecond arrangement configured to determine the information regarding theat least one structure, wherein the at least one second arrangement isfurther configured to (i) determine a distance from at least one portionof the at least one structure to the at least one first arrangementbased on the information, and (ii) track at least one peak of at leastone signal of the information which includes image data regarding the atleast one structure, wherein the at least one computer secondarrangement is further configured to, as a function of the distance andthe at least one peak, control an optical radiation path length of athird optical radiation which is at least one of: i. at least one oftransmitted to or received from the at least one structure, or ii. atleast one of transmitted to or received from the reference, and whereinthe at least one computer second arrangement is further configured todetermine the distance using at least one of a zero-th order procedure,a first derivative procedure, a second derivative procedure, adetermination of a probability distribution function statistics or afitting procedure as applied to the information which is based on thedistance and the at least one peak.
 2. The apparatus according to claim1, wherein the at least one portion is a surface of the at least onestructure.
 3. The apparatus according to claim 1, wherein the at leastone structure includes an anatomical structure.
 4. The apparatusaccording to claim 1, wherein the at least one second arrangement isfurther configured to (i) obtain information associated with the atleast one structure associated the controlled optical radiation pathlength, and (ii) generate at least one image of the at least one portionas a function of the information.
 5. The apparatus according to claim 1,wherein the at least one second arrangement is further configured tocontrol an optical radiation path length while an image of the at leastone portion is generated.
 6. The apparatus for obtaining informationassociated with at least one structure, comprising: at least one opticalcoupler first arrangement configured to receive at least one firstoptical radiation from the at least one structure and at least onesecond optical radiation from a reference; and at least one computersecond arrangement configured to determine the information regarding theat least one structure as a function of depth thereof, wherein the atleast one second arrangement is further configured to determine adistance from at least one portion of the at least one structure to theat least one first arrangement based on the information, wherein the atleast one computer second arrangement is further configured to, as afunction of the distance, control an optical radiation path length of athird optical radiation which is at least one of: i. at least one oftransmitted to or received from the at least one structure, or ii. atleast one of transmitted to or received from the reference, wherein theat least one computer second arrangement is further configured todetermine the distance using at least one of a zero-th order procedure,a first derivative procedure, a second derivative procedure, adetermination of a probability distribution function statistics or afitting procedure as applied to the information which is based on thedepth, wherein the at least one second arrangement is further configuredto determine a length from the at least one portion based on the atleast one first optical radiation, and wherein the at least one secondarrangement is further configured to, as a function of the length,control a focal distance of the at least one third optical radiationwhich is at least one of transmitted to or received from the at leastone structure.
 7. The apparatus according to claim 1, wherein the atleast one second arrangement is further configured to determine anoptical radiation penetration depth within the at least one portionbased on at least one of the first optical radiation or the secondoptical radiation, wherein the at least one second arrangement isfurther configured to, as a function of the penetration depth, controlthe optical radiation path length range.
 8. An apparatus for obtaininginformation associated with at least one structure, comprising: at leastone lens first arrangement configured to receive at least one firstoptical radiation from the at least one structure; and at least onecomputer second arrangement configured to determine a length from atleast one portion of the at least one structure based on the at leastone first optical radiation, wherein the at least one computer secondarrangement is further configured to, as a function of the length,control a focal distance of at least one of at least one second opticalradiation which is at least one of transmitted to or received from theat least one structure.
 9. The apparatus according to claim 8, whereinthe at least one portion is a surface of the at least one structure. 10.The apparatus according to claim 8, wherein the at least one secondarrangement is further configured to determine a distance from the atleast one portion based on at least one of the at least one firstoptical radiation or at least one third electromagnetic radiationreceived from a reference, wherein the at least one second arrangementis further configured to, as a function of the distance, control anoptical radiation path length of the at least one second opticalradiation which is at least one: i. at least one of transmitted to orreceived from the at least one structure, or ii. at least one oftransmitted to or received from the reference.
 11. The apparatusaccording to claim 10, wherein the at least one second arrangement isfurther configured to determine the distance using at least one of azero-th order procedure, a first derivative procedure, a secondderivative procedure, a determination of a probability distributionfunction statistics or a fitting procedure.
 12. The apparatus accordingto claim 8, wherein the at least one structure includes an anatomicalstructure.
 13. The apparatus according to claim 10, wherein the at leastone second arrangement is further configured to (i) obtain informationassociated with the at least one structure associated the controlledoptical radiation path length, and (ii) generate at least one image ofthe at least one portion as a function of the information.
 14. Theapparatus according to claim 8, wherein the at least one secondarrangement is further configured to control an optical radiation pathlength while an image of the at least one portion is generated.
 15. Theapparatus according to claim 8, wherein the at least one firstarrangement is further configured to receive at least one third opticalradiation from a reference, and wherein at least one second arrangementis configured to determine the distance from the at least one portionbased on at least one of the first optical radiation or the thirdelectromagnetic radiation.
 16. The apparatus according to claim 10,wherein the at least one second arrangement is further configured todetermine an optical radiation penetration depth within the at least oneportion based on at least one of the first electromagnetic radiation orthe third optical radiation, wherein the at least one second arrangementis further configured to, as a function of the penetration depth,control the optical radiation path length range.
 17. A method forobtaining information associated with at least one structure,comprising: receiving at least one first optical radiation from the atleast one structure and at least one second optical radiation from areference; determining the information regarding the at least onestructure as a function of depth thereof; determining a distance from atleast one portion of the at least one structure based on theinformation; tracking at least one peak of at least one signal of theinformation which includes image data regarding the at least onestructure; as a function of the distance and the at least one peak,controlling an optical radiation path length of at least one thirdoptical radiation which is at least one of: i. at least one oftransmitted to or received from the at least one structure, or ii. atleast one of transmitted to or received from the reference; and using acomputer arrangement, determining the distance using at least one of azero-th order procedure, a first derivative procedure, a secondderivative procedure, a determination of a probability distributionfunction statistics or a fitting procedure as applied to the informationwhich is based on the distance and the at least one peak.
 18. A methodfor obtaining information associated with at least one structure,comprising: receiving at least one first optical radiation from the atleast one structure; determining a length from at least one portion ofthe at least one structure based on the at least one first opticalradiation; and using a computer arrangement, as a function of thelength, controlling a focal distance of at least one of at least onesecond optical radiation which is at least one of transmitted to orreceived from the at least one structure.